Invalid level of header

I tested this markdown using this javascript validator for getting the content of header.

#1

ああああああああああああああ

## 1.1

あああああああああああああ

### 1.1.1

あああああああああああああ

#2

ああああああああああああああ

## 2.1

ああああああああああ。

function validateSection(section){
    console = {
        log:print
    };


    console.log(section.getLevel())
    console.log(section.getHeaderContent(0))
}

The result of this test was as below. What is the section of level 0 ?

0
Sentence{links=[], lineNumber=0, content='', startPositionOffset=0, isFirstSentence=false, tokens=[], offsetMap=[]}
1
Sentence{links=[], lineNumber=1, content='1', startPositionOffset=2, isFirstSentence=true, tokens=[], offsetMap=[LineOffset(1, 2)]}
2
Sentence{links=[], lineNumber=5, content='1.1', startPositionOffset=3, isFirstSentence=true, tokens=[], offsetMap=[LineOffset(5, 3), LineOffset(5, 4), LineOffset(5, 5)]}
3
Sentence{links=[], lineNumber=9, content='1.1.1', startPositionOffset=4, isFirstSentence=true, tokens=[], offsetMap=[LineOffset(9, 4), LineOffset(9, 5), LineOffset(9, 6), LineOffset(9, 7), LineOffset(9, 8)]}
1
Sentence{links=[], lineNumber=13, content='2', startPositionOffset=2, isFirstSentence=true, tokens=[], offsetMap=[LineOffset(13, 2)]}
2
Sentence{links=[], lineNumber=17, content='2.1', startPositionOffset=3, isFirstSentence=true, tokens=[], offsetMap=[LineOffset(17, 3), LineOffset(17, 4), LineOffset(17, 5)]}

[kongou-ae]

@takahi-i , Thank you for your detailed explanation. I understood the reason of the level zero section 0. I am looking forward to be implemented #648.

[takahi-i]

Hello @kongou-ae, I merged the fix for this problem in #648. Hopefully the fix works for you. Let me know when the fix does not work adding another issue or reopen this issue.